Add .cost() to notification model

In the V2 API, the GET response for an individual notification
returns a 'cost' value, which we can get by multiplying the
billable units by the per-message rate of the supplier who
sent the message.
Any notifications with billable units > 0 but without a
corresponding `ProviderRates` entry will blow up the application,
so make sure you've got one.

tests/app/test_model.py

@@ -1,9 +1,7 @@
-import uuid
+import pytest
 from datetime import datetime
 
-import pytest
-
-from app import DATETIME_FORMAT
+from tests.app.conftest import sample_notification, sample_provider_rate
 from app.models import (
     Notification,
     ServiceWhitelist,
@@ -37,3 +35,29 @@ def test_should_build_service_whitelist_from_email_address(email_address):
 def test_should_not_build_service_whitelist_from_invalid_contact(recipient_type, contact):
     with pytest.raises(ValueError):
         ServiceWhitelist.from_string('service_id', recipient_type, contact)
+
+
+@pytest.mark.parametrize('provider, billable_units, expected_cost', [
+    ('mmg', 1, 3.5),
+    ('firetext', 2, 5),
+    ('ses', 0, 0)
+])
+def test_calculate_cost_from_notification_billable_units(
+        notify_db, notify_db_session, provider, billable_units, expected_cost
+):
+    provider_rates = [
+        ('mmg', datetime(2016, 7, 1), 1.5),
+        ('firetext', datetime(2016, 7, 1), 2.5),
+        ('mmg', datetime.utcnow(), 3.5),
+    ]
+    for provider_identifier, valid_from, rate in provider_rates:
+        sample_provider_rate(
+            notify_db,
+            notify_db_session,
+            provider_identifier=provider_identifier,
+            valid_from=valid_from,
+            rate=rate
+        )
+
+    notification = sample_notification(notify_db, notify_db_session, billable_units=billable_units, sent_by=provider)
+    assert notification.cost() == expected_cost